1. Field of the Invention
This invention is in the field of manufacturing rotary ball display devices and in particular relates to a method for producing rotary ball display devices of improved quality.
2. Description of the Prior Art
Previously proposed rotary ball display devices included a number of display ball members in which one hemispherical surface of each ball was colored white and the other hemispherical surface was colored, for example, black by an insulating color layer. The spherical cavities were then filled with an organic liquid having a predetermined density and a high resistivity, for example, toluene and were supported so as to be freely rotatable. In this type of device, the nature of the organic liquid and the mutual charged condition of the different colored portions of the display ball members were made different so as to be opposite in polarity in the liquid. Accordingly, when an external electric field was applied to the display ball members in response to the material to be displayed, the predetermined colored side of the display ball member could be rotated toward the side of the viewer so as to make the desired display pattern.
Since the colored surface layer of the display ball member should be adherent to the ball it is considered advantageous that the color layer substance be an inorganic substance rather than an organic one. Inorganic coloring substances, however, generally have a considerably higher refractive index than the organic liquid. For example, the refractive index of toluene is about 1.49 while that of the blackish brown inorganic coloring substance antimony trisulfide (Sb.sub.2 S.sub.3) is 3.0. Accordingly, if a rotary ball display device having a colored layer of such a high refractive index is supported in the organic liquid, an external light is reflected on an interface or boundary surface between the surface of the colored layer and the liquid, thereby inhibiting the formation of the required color display pattern. Consequently, a clear display with a high contrast cannot be obtained.
There have been previously proposed several methods for manufacturing the rotary ball display device with the aforementioned arrangement, particularly a display device which includes a display ball member together with an organic liquid. For example, it has been proposed to mix a number of display ball members into an elastomer, mold and harden the elastomer into the form of a sheet, and immerse this elastomer sheet into an organic solvent where it is swollen to provide cavities about the respective display ball members and producing a sheet shaped display member in which each of the cavities is filled with an organic liquid having high resistivity. However, it has been impossible to control the arrangement and packing density of the display ball members to a sufficiently high value so that a display of high quality, high resolution, and high contrast could not be achieved. Moreover, the display device of such an arrangement poses problems in connection with the selection of a usable organic liquid, stability upon changes in temperature, non-uniformity of dimensions of the cavities, and the like.
There have also been methods involving doctor blade application and slurry application to produce the ball member. The former technique requires strict adherence to critical conditions so that mass production capabilities are limited and the packing density of the ball members cannot be increased. The slurry method is inferior because it does not produce a sufficiently high packing density to achieve high definition display.